A method for activating a conventional hydrogen generator used for supplying hydrogen to a fuel cell will be described, referring to FIG. 8. FIG. 8 is a schematic view showing a configuration of the conventional hydrogen generator.
The hydrogen generator shown in FIG. 8 comprises a material supply unit 1 and a water supply unit 2, each being connected to a reforming unit 3 filled with a reforming catalyst. A raw material supplied from the material supply unit 1 is reformed in the reforming unit 3; the resultant reformed gas flows into a shifting unit 4 filled with a shifting catalyst to be shifted; and the resultant shifted gas flows out of the shifting unit 4 and into a purifying unit 5 filled with a CO removing (purifying) catalyst to be purified The resultant purified gas flows out of the purifying unit 5 and, as a generated gas, passes along through a three-way valve 6 to be supplied to a fuel cell 7, or, in some cases, is led to a burner 8 arranged in the vicinity of the reforming unit 3. The hydrogen generator shown in FIG. 8 comprises the reforming unit 3, the shifting unit 4 and the purifying unit 5, but in some cases, the hydrogen generator may comprise only the reforming unit, or may comprise only the reforming unit 3 and the shifting unit 4. The burner 8 comprises a combustion chamber 8′, and a fuel supply unit 9 and an air supply unit 10 for supplying air for combustion are further arranged. A combustion gas that generates in the burner 8 is discharged from out of an outlet 11 arranged on the reforming unit 3.
At the time of start-up of the hydrogen generator having the configuration as thus described, the generated gas left in the purifying unit 5 and air for combustion from the air supply unit 10 are supplied to the burner 8, and, while an igniting operation is being conducted with an ignition device (not shown), a fuel is supplied from the fuel supply unit 9 to the burner 8, to form a fire. After confirming that the fire is in a stable state, the raw material is supplied from the material supply unit 1 to the reforming unit 3 and passes along through the reforming unit 3, the shifting unit 4 and the purifying unit 5 to give a generated gas, which combusts with the fuel supplied from the fuel supply unit 9 in the burner 8, to heat the reforming unit 3.
Subsequently, the amount of the fuel supplied from the fuel supply unit 9 was gradually reduced and the supply was eventually discontinued, making the generated gas, obtained from the raw material supplied from the material supply unit 1, singly form the fire, which increases temperatures of the reforming unit 3, the shifting unit 4 and the purifying unit 5 to become the optimum temperature conditions thereof, so that the hydrogen generator is activated.
At that time, the amount of air supplied from the air supply unit 10 is adjusted corresponding to the amount of the raw material supplied from the material supply unit 1. In this case, however, because the amount of air corresponds only with the amount of the raw material and thus does not sufficiently correspond with the amount of the combustible gas in the generated gas which actually combusts in the burner 8, the air becomes excess or deficient, which may cause deterioration of characteristic of a combustion exhaust gas and an unstable state of combustion.
The composition of the generated gas and the flow rate of each constituent thereof are determined by the states of the reactions of the catalysts contained in the reaction units such as the reforming unit 3, the shifting unit 4 and the purifying unit 5, which are, for example, the temperatures of the catalysts. In a case where methane is used as the raw material gas, for example, reforming reactions in the reforming unit are primarily represented by the formula (1) and the formula (2) below:CH4+2H2O→4H2+CO2  (1)CH4+H2O→3H2+CO  (2)
When a temperature of the reforming catalyst is too low to induce the reforming reaction, the generated gas supplied from the hydrogen generator to the burner 8 is methane which was supplied as the raw material. If the temperature of the reforming catalyst rises to the extent that the reforming reaction sufficiently occurs, however, the reformed gas sent out of the reforming unit 3 are mainly hydrogen and carbon dioxide or carbon monoxide, according to the formula (1) and the formula (2) above, and the total flow rate of the reformed gas is four to five times as high as the flow rate of the supplied methane. Until the temperature of the reforming catalyst rises sufficiently, the composition of the generated gas and the flow rate of each constituent thereof have intermediate values, and with occurrence of subsequent reactions in the shifting unit 4 and the purifying unit 5, the composition of the generated gas and the flow rate of each constituent thereof further change depending on the temperatures of these reaction units.
As thus described, changes in the composition of the generated gas and the like depending on the temperature of each reaction unit brings a change in the amount of the combustible gas in the generated gas. This has raised a problem that adjustment of the amount of air by corresponding to the amount of the supplied raw material causes excess or deficiency of air, making it difficult to consistently maintain a favorable state of combustion in the burner 8. Particularly in a case where the temperature of the reforming catalyst is around 400° C., a reaction rate goes up by several tens of percent per a temperature rise of 10° C., leading to a sudden increase in the flow rate of the gas sent out of the reforming unit 3, which pushes a large amount of the combustible gas present in the shifting unit 4 and the purifying unit 5 into the burner 8. Determination of the amount of air by corresponding to the amount of the supplied raw material may, therefore, cause a shortage of air to a considerable degree, making the fire in the burner 8 prone to be unstable and possibly leading to extinguishment.
It is an object of the present invention, then, to solve the above problems and provide a hydrogen generator, where a generated gas therefrom stably combusts in a burner and which is thereby excellent in operation and high in convenience.
Next, a conventional fuel cell system using the above-mentioned hydrogen generator and a fuel cell will be described: FIG. 9 is a schematic view showing a configuration of the conventional fuel cell system. In a fuel cell 101 in the fuel cell system shown in FIG. 9, an air electrode 102 and a fuel electrode 103 are arranged with a polymer electrolyte membrane 104 interposed therebetween, and the upstream side of the air electrode 102 is connected to a blower (an air supply unit) 105 for supplying air.
A hydrogen generator 106 is provided with a raw material X such as natural gas or methanol and water Y required for a steam reforming reaction, and generates a hydrogen-rich generated gas (a reformed gas) G. It is to be noted that the hydrogen generator 106 in FIG. 9 comprises only a reforming unit.
The generated gas G is supplied to the fuel electrode 103 in the fuel cell 101 via a switching valve 107 and flows through a predetermined flow path, which is in contact with the fuel electrode 103, toward the downstream side. At that time, just the required amount of hydrogen in the generated gas G is consumed due to an electrode reaction and the remaining, unreacted gas in the fuel cell 101 flows through a gas flow path 123′ to be supplied to the burner 109 as an off-gas G′. When the generated gas G is not supplied to the fuel electrode 103, the gas passes through the switching valve 107 and flows through a gas flow path 123 to be supplied to the burner 109.
The generated gas G or the off-gas G′, supplied to the burner 109, combusts after being mixed with air supplied from a fan (an air supply unit) 110, and forms a fire 111 in a combustion chamber 108, to heat the hydrogen generator 106 with a combustion gas.
A state of the fire 111 in the combustion chamber 108 is detected by an ion-current which flows when a predetermined voltage is applied to the fire 111. A fire detecting unit 112 is constituted of a heat-resistant conductor 113 arranged such that it comes in contact with the fire 111, a direct current power source 114 for applying a predetermined voltage to the conductor 113 and to the burner 109 via the fire 111, an electric resistance 115 for converting a current flowing in the fire 111 into a voltage and a voltage detecting unit 116 for detecting a voltage across the electric resistance 115. This fire detecting unit 112 can detect a combustion state of the fire 111 such as ignition or extinguishment.
In such a conventional fuel cell system, concentrations of hydrocarbon in the generated gas G and the off-gas G′ are significantly low since hydrocarbon in the raw material X has been converted into hydrogen due to a steam reforming reaction. The low concentration of hydrocarbon lowers a concentration of ions in the fire 111, which also decreases the value of the current flowing in the fire 111 and thereby lowers the voltage across the electric resistance 115. That is to say, a voltage to be detected by the fire detecting unit 112 is lowered, raising the problem of difficulty in determining the combustion state at the time of ignition and fire extinguishment.
It is an object of the present invention, accordingly, to solve the above problem and provide a hydrogen generator which ensures determination of ignition and fire extinguishment of a burner for heating the hydrogen generator and can be safely operated, and a fuel cell system comprising the above-mentioned hydrogen generator.